Multi-die power modules are classically composed of several parallel connected power dies and are used for increasing the current capability over that of a single power die.
For example, a three-phase converter is composed of four parallel power dies per switch, giving twenty four power dies in total.
Emerging devices technologies, such as SiC (Silicon Carbide) and GaN (Gallium Nitride) Transistors, are typically realized in high current density, small power dies due to limitations of yield and cost of wafer substrate.
In order to realize higher power SiC-based modules, a multitude of parallel connected SiC dies is necessary. Unlike parallel connected modules, parallel connected dies constitute a single switch that ideally commutates the same load current.
However, regardless of the type of die used, i.e. diodes or voltage-driven switch, e.g. MOSFETs (Metal Oxide Semiconductor Field Effect Transistor), characteristics exist within the dies that limit the balanced sharing of the load current both statically and dynamically. The incremental addition of each parallel die does not result in full utilization of the die, and thus, more dies are needed in parallel to achieve a given current rating, thereby increasing the overall costs and physical surface area of the power module.
Due to variation of electrical characteristics of power device at manufacture, the currents are not equally shared across dies, and especially during switching transitions.
One key electrical characteristic for the switching transient is the threshold voltage. For normally-on die, when the voltage across source and gate of a die is below the threshold voltage of the die, the die is in OFF state, i.e. not conducting. Above the threshold voltage, the die is in ON state, i.E. starts conducting current between the source and the drain of the die. When the threshold voltages are not perfectly aligned across parallel dies controlled with same gate to source voltage, currents are not equally shared across the dies during the transient. As a result, switching losses can significantly differ across parallel dies, leading to undesired temperature imbalance across dies, and eventually, early ageing of most stressed die.